Motor vehicles with exhaust treatment devices, in which a liquid additive for exhaust gas cleaning is added, are widely used. Exhaust treatment devices in which the method of selective catalytic reduction (SCR method, SCR=Selective Catalytic Reduction) is performed are encountered particularly frequently. With this method, nitrogen oxide compounds in the exhaust gas with ammonia are reduced to harmless substances such as nitrogen, water and CO2. Ammonia is not normally stored directly in a motor vehicle, but in the form of a liquid additive, which can be stored in a tank. Such a liquid additive for exhaust gas cleaning is then converted to ammonia outside the exhaust gas in a reactor provided for the purpose or within the exhaust gas in an exhaust treatment device. Particularly frequently, a urea-water solution is used as the liquid additive. A urea-water solution with a urea content of 32.5% by weight is available under the trade name AdBlue®.
It is frequently necessary to obtain information about the level in a tank for liquid additive. Information about the level can be used to determine suitable points in time for filling the tank. Moreover, it can be prevented that the tank is completely emptied during operation.
Ultrasonic level sensors have already been proposed for determining the level in a tank for liquid additive. Ultrasonic level sensors normally include an ultrasonic transmission unit and an ultrasonic receiving unit. The ultrasonic transmission unit transmits an ultrasonic wave, which is reflected at a liquid surface within the tank and from there returns to the ultrasonic level sensor and is received by the ultrasonic receiving unit of the ultrasonic level sensor. The propagation speed of the ultrasonic wave in the liquid in the tank is either known or it is determined with a reference measurement. The level in the tank is computed from the propagation time of the ultrasonic wave from the ultrasonic level sensor to the liquid level and back to the ultrasonic level sensor and the propagation speed of the ultrasonic wave in the liquid.
An advantage of ultrasonic level sensors is that they do not include any movable parts. Moreover, with ultrasonic level sensors it is possible to measure the level in a tank with the same sensor design even for different tank heights. Differences with regard to the possible level in the tank only have to be stored in an evaluation unit with which the ultrasonic waves received by the ultrasonic receiving unit are evaluated. Ultrasonic waves received by the ultrasonic receiving unit, which originate from the ultrasonic waves emitted by the ultrasonic transmission unit and were, for example, reflected at the liquid surface, are also referred to below as signals or as response signals, which are received by the ultrasonic receiving unit or by the ultrasonic level sensor.
Two different arrangements of ultrasonic level sensors in a tank are known. According to a known arrangement, ultrasonic waves are sent from an upper point to a liquid level in the tank and are reflected from there back up to the ultrasonic level sensor. In another known arrangement, ultrasonic waves are sent from an ultrasonic level sensor on the floor of the tank through the liquid to the liquid level in the tank, and are reflected at the liquid level back to the ultrasonic level sensor on the floor of the tank.
It has already been explained above that a reference measurement can be performed for determining the propagation speed of ultrasonic waves in a liquid. It is known to determine the propagation time of an ultrasonic wave along a known measuring distance in the liquid for the reference measurement. A measuring distance can be implemented, for example, with at least one reference surface at which the ultrasonic wave is reflected. Preferably, the position of the reference surface or the distance between two reference surfaces is accurately known. A speed of travel of the ultrasonic wave in the liquid is determined using the propagation time of the ultrasonic wave to the reference surface or using a difference of the propagation times of the ultrasonic waves to a plurality of (e.g. two) reference surfaces.
In carrying out such a reference measurement it is problematic that the measuring distance given by the at least one reference surface must be disposed fully within the liquid. This is especially problematic with changing levels and fill heights of the liquid in the tank. It is therefore known to arrange the reference surfaces such that the measuring distance for determining the speed of travel in the ultrasonic wave is horizontal, preferably close to the bottom of the tank. This enables it to be guaranteed that a propagation time measurement can be performed even for very low levels. It is a disadvantage that the horizontal arrangement of a measuring distance takes up a relatively large installation space at the bottom of the tank.